The present invention relates generally to motor systems and, more particularly, to a system and method for controlling the effects of motor back electromotive force (EMF) on a motor drive unit. The present invention utilizes high-speed semiconductor switches to short the motor leads in the event of a fault condition. As such, the generative potential of the motor is contained and the motor drive unit is protected from the high voltage produced by the motor during high speeds.
There are a variety of applications that require a motor to deliver constant power over a wide operational range. Permanent magnet (PM) synchronous motors have often been utilized in such applications because, by properly adjusting the combination of magnetic saliency and permanent magnet flux in the PM motor design, the resulting constant power speed ratio can reach values of ten or higher.
However, when designing and/or utilizing PM motors, there are several factors that must be taken into consideration for a practical application requiring a wide constant power speed ratio. For example, in a typical PM motor drive unit configuration utilizing a traditional six-switch, full-bridge inverter to excite a three-phase PM motor, when the motor is rotating at high speed, the amplitude of the line-to-line back EMF generated by the spinning PM rotor magnets may significantly exceed the source voltage supplied over the DC link. Generally, the effects of the back EMF generated during high-speed operation are automatically limited by the applied DC link voltage. That is, the inverter switches operate in a controlled, flux-weakening mode that serves to keep the back EMF of the motor from injecting a potentially damaging current flow back toward the inverter.
However, should a fault arise while the motor is operating at high-speeds that causes the gate excitation to be removed from the controlled inverter switches, the generative potential of the motor will no longer be controlled. In the case of such a fault causing the inverter to “shut-down”, the high amplitude of the motor back EMF causes current to flow back through the freewheeling diodes of the inverter to the DC link. That is, at high speeds, the PM motor acts as a high voltage generator and will deliver power back toward the motor drive unit unless controlled. The resulting “reverse” current flow continues until the rotor speed has been sufficiently reduced and the current flow is extinguished.
This “reverse” current flow and the associated high voltage from PM motor may damage the capacitors of the DC link and/or switches of the inverter. Therefore, the PM motor has the potential to damage or destroy the power electronics of the motor drive unit and may even create an unsafe condition in high kinetic energy applications.
Various attempts have been made to control the generative potential of a PM motor under fault conditions. For example, large dynamic braking kits have been used to brake or stop the motor upon detection of a fault. However, such braking systems require additional system design and maintenance. Furthermore, while these braking systems reduce the impact of a fault, they are unable to react to fault conditions quickly enough to stop the motor from injecting the “reverse” current flow back toward the motor drive unit. In particular, the voltage generated by the motor at high speeds following a fault condition has a time constant between fractions of millisecond to a few milliseconds. As such, the components of the motor drive unit may still be damaged by being subjected to a significant voltage increase that occurs while the braking system attempts to control the motor.
Alternatively, high voltage components have been used that are rated to withstand the maximum motor terminal voltage under uncontrolled generative operation. However, the cost of high voltage DC link capacitors and high voltage inverter switches significantly increases the cost of the drive design. Additionally, the use of high voltage silicon switches in the inverter significantly increases the losses incurred along the inverter and; thereby, decreases the power density and overall efficiency of the motor drive unit.
Therefore, it would be desirable to have a system and method to protect a motor drive unit from the generative potential of a PM motor under fault conditions.